Internal combustion engine having exhaust gas recirculation loop with catalyzed heat exchanger for steam reformation

ABSTRACT

A method of providing recirculated exhaust to an internal combustion engine. The engine has an EGR (exhaust gas recirculation loop) which may be a high pressure loop, a low pressure loop, or a dedicated cylinder loop. A catalyzed heat exchanger on the EGR loop has a steam reformation catalyst and a heat exchanger. The heat exchanger uses hot exhaust gas from the main exhaust line to heat the catalyst. A methane fuel source adds methane to the recirculated exhaust stream before it enters the catalyst, and the catalytic reaction increases the amount of hydrogen and carbon monoxide in the recirculated exhaust.

TECHNICAL FIELD OF THE INVENTION

This invention relates to internal combustion engines with exhaust gas recirculation, and more particularly to improving the composition of the recirculated exhaust.

BACKGROUND OF THE INVENTION

For many internal combustion engines, their engine control strategy has three important parameters: spark timing (or fuel injection timing in a diesel engine), the exhaust gas recirculation (EGR) rate and the air/fuel ratio (AFR).

To implement EGR, a fraction of the exhaust gas is recycled from the exhaust system back to the intake system. The recirculated exhaust gas is mixed with the fresh fuel-air mixture before entering the cylinders.

EGR can be implemented in internal and external configurations. External configurations can be implemented as either a high pressure or low pressure loop, depending on whether the recirculated exhaust is introduced to the intake post compressor (high pressure loop) or pre-compressor (low pressure loop).

EGR has a long history of use in both diesel and spark-ignited engines for reduction of NOx emissions. It affects combustion in several ways. The combustion is cooled by the presence of exhaust gas, that is, the recirculated exhaust gas absorbs heat. The reduction of peak gas temperatures in the combustion chamber reduces the production of NOx.

One approach to implementing EGR is with one or more dedicated EGR cylinders. In a “dedicated EGR” engine, the one or more dedicated cylinders are used to generate exhaust gas, all of which is dedicated to recirculation. With dedicated EGR, the quality of the recirculated exhaust can be improved with in-cylinder reforming of gasoline to hydrogen (H₂) and carbon monoxide (CO). Subsequent combustion of this exhaust, mixed with the fresh air intake to produce the intake charge, is thereby enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:

FIG. 1 illustrates an example of an internal combustion engine having a dedicated EGR cylinder and a steam reformation catalyst in accordance with the invention.

FIG. 2 illustrates a catalyzed heat exchanger in accordance with the invention.

FIG. 3 illustrates an example of an internal combustion engine having a high pressure loop for EGR and a steam reformation catalyst in accordance with the invention.

FIG. 4 illustrates an example of an internal combustion engine having a low pressure loop for EGR and a steam reformation catalyst in accordance with the invention.

FIG. 5 illustrates how conversion of methane improves as temperature increases, for membrane and packed bed reactors.

FIGS. 6 and 7 illustrate expected H₂ and CO percentages, respectively, in the exhaust stream as a result of use of the catalyzed heat exchanger.

DETAILED DESCRIPTION OF THE INVENTION

The following description is directed to systems and methods for improving the operation of an internal combustion engine having exhaust gas recirculation (EGR). A steam reforming catalyst with a built-in heat exchanger (referred to herein as a “catalyzed heat exchanger”) is installed in the EGR loop for the purpose of increasing the H2 and CO content of the recirculated exhaust.

The catalyzed heat exchanger may be used with either a high pressure loop or low pressure loop EGR system. It may also be used with a dedicated EGR system.

Catalyzed Heat Exchanger with Dedicated EGR

FIG. 1 illustrates an internal combustion engine 100 having four cylinders 101. One of the cylinders is a dedicated EGR cylinder, identified as cylinder 101 d. Cylinder 101 d has an exhaust port that opens only to an EGR loop 110. The other cylinders 101 produce exhaust that exits the engine via the main exhaust line 119 and one or more exhaust aftertreatment devices 120.

In the example of this description, engine 100 is spark-ignited, with each cylinder 101 having an associated spark plug, and its “normal” air-fuel ratio (that of the cylinders 101 other than the dedicated EGR cylinder 101 d) is stoichiometric. Engine 100 may use various types of fuels, such as gasoline, and the invention described herein is particularly useful with natural gas fueled engines.

A feature of dedicated EGR is that the composition of the dedicated EGR exhaust gas may be controlled to be different from that of the exhaust of the non-dedicated cylinders. For example, the dedicated EGR cylinder 101 d may be operated rich of stoichiometric to provide EGR that improves combustion on all cylinders. Thus, in a dedicated EGR engine, the dedicated and non dedicated cylinders may receive different amounts of fuel. This is in contrast to a conventional engine, in which the cylinders are typically fueled such that all cylinders receive the same amount of fuel.

If a dedicated EGR cylinder is run rich of stoichiometric A/F ratio, a significant amount of hydrogen (H₂) and carbon monoxide (CO) may be formed in the exhaust gas of the dedicated EGR cylinder. The benefits of H₂ (for both increased knock and EGR tolerance) are greater than the benefits of CO. Thus, it is desired to produce more H₂ than CO.

The dedicated EGR cylinder 101 d has all of its exhaust recirculated back to the intake manifold 102. The exhaust of the other three cylinders 101 (referred to herein as the “main” or “non dedicated” cylinders) is directed to the main exhaust line 119 via an exhaust manifold 103.

Engine 100 is equipped with a turbocharger, comprising a compressor 104 a and a turbine 104 b. Although not explicitly shown, all cylinders 101 have some sort of fuel delivery system for introducing fuel into the cylinders. This fuel delivery system can be fumigated, port injected, or direct injected.

The EGR loop 110 joins the intake line downstream the compressor 104 a. A mixer 105 mixes the EGR and fresh air, and a cooler 106 reduces the temperature of the intake charge. A throttle 107 controls the amount of intake (fresh air and EGR) into the intake manifold 102. Various means, such as variable valve timing, valves, etc. (not shown), may be used to control EGR flow.

The “EGR fraction” in the example of FIG. 1 is 25%, with one of four cylinders 101 being a dedicated EGR cylinder. In other embodiments, there may be a different number of engine cylinders, and/or there may be more than one dedicated EGR cylinder. In general, in a dedicated EGR engine configuration, the exhaust of a sub-group of cylinders is routed back to the intake of all the cylinders, thereby providing EGR for all cylinders.

After entering the cylinders 101, the fresh-air/EGR mixture is ignited and combusts. After combustion, exhaust gas from each main cylinder 101 flows through its exhaust port and into exhaust manifold 103. From the exhaust manifold 103, exhaust gas then flows through turbine 104 b, which drives compressor 104 a.

After turbine 104 b, exhaust gas flows to an exhaust aftertreatment device 120 via the main exhaust line 119, to be treated before exiting to the atmosphere. Because only stoichiometric exhaust leaves the engine, the exhaust aftertreatment device 120 may be a three way catalyst.

To control the air-fuel ratio, exhaust gas may be sampled by an exhaust gas oxygen (EGO) sensor. Both the main exhaust line 119 and the EGR loop 110 may have a sensor (identified as 166 a and 166 b).

An EGR control unit (not shown) has appropriate hardware (processing and memory devices) and programming for performing the methods described herein. In addition, the control unit may perform other tasks, such as overall EGR control, and may be integrated with a comprehensive engine control unit.

A catalyzed heat exchanger (reactor) 180 is catalyzed to act as a steam-methane reformation catalyst, and is installed in-line on the EGR loop 110 to process the EGR exhaust. As explained below, heat exchanger 180 uses exhaust from the main exhaust line 119 to raise the temperature of the EGR exhaust.

A fuel injector 185 injects a methane source, such as natural gas or other hydrocarbon fuel, into the EGR stream upstream of catalyzed heat exchanger 180. In theory, any hydrocarbon fuel may be steam reformed. Specific examples are gasoline, diesel, methane, propane, and ethanol.

Fuel injector 185 is located downstream of the EGR take-off point from dedicated EGR cylinder 101 d, and upstream of catalyzed heat exchanger 180.

If engine 100 is fueled by natural gas, the methane source may be from the same fuel reservoir as used for the engine. In engines fueled with fuels other than natural gas, the methane source may be stored in an on-board reservoir (not shown) separate from that of the engine fuel.

Steam reforming of natural gas, sometimes referred to as steam methane reforming, is conventionally used commercially to produce bulk hydrogen. At high temperatures (500-1100° C.) and in the presence of a metal-based catalyst, steam reacts with methane (CH₄) to yield carbon monoxide (CO) and hydrogen (H₂). Steam reformation catalysts are typically formulated with nickel as the active metal, although other active components may be utilized.

The steam reformation reaction is endothermic, meaning energy is consumed during the reaction. Heat must be supplied to the process for the reaction to proceed. The lower heating value (or energy content) of the formed H₂ and CO mixture is greater than the lower heating value of the consumed CH₄. This allows for conversion of thermal energy to chemical energy in the form of H₂ and CO, as shown in the following equation. CH₄ (g)+H₂O (v)

CO+3H₂

As stated above, for purposes of recirculated exhaust, the benefits of H₂ are greater than the benefits of CO. Therefore, it would be desired for catalyzed heat exchanger 180 to produce more H₂ than CO. The relative production of H₂ and CO is pre-determined by the H:C ratio of the fuel used in the reforming process. Also, more H₂ and CO can be generated using the steam reforming reaction if more thermal energy is available to drive the reactor.

To this end, catalyzed heat exchanger 180 is configured to transfer thermal energy from the main exhaust stream to the EGR stream. In other words, catalyzer heat exchanger 180 is a combined catalyst/heat exchanger system. For the four-cylinder dedicated EGR engine of FIG. 1, catalyzed heat exchanger 180 has the potential to increase the thermal energy of the EGR stream by up to a factor of three.

In one embodiment, catalyzed heat exchanger 180 comprises a high temperature heat exchanger, which is coated with a catalyst material for the steam reforming reaction. The main exhaust stream from main exhaust line 119 is used to keep the surface of the catalyst material at the same temperature as the main exhaust stream. The surface temperature of the catalyst controls the chemical reaction rates, with the exhaust from exhaust manifold 103 maintaining a constant catalyst surface temperature leading to a high yield of H₂ and CO.

FIG. 2 is a cross-sectional view of an example of a catalyzed steam reformer 180. A housing 241 contains both a heat exchanger 243 and a catalyzed reactor 245. In the example of FIG. 2, exhaust from the main exhaust line 119 enters an outer heat exchanger jacket 241 and circulates around the surface of catalyst reactor 245. The heat exchanger jacket 241 transfers thermal energy from the main exhaust stream to the EGR stream as thermal energy is converted in the EGR stream via the steam reforming reaction. This heat exchange maintains high catalyst surface temperature in the EGR stream as the EGR stream flows through the catalyzed reactor 245 and during the steam reforming reaction.

Catalyst 245 may be implemented with catalyzed pellets, honeycomb surfaces, or any other configuration used for catalytically treating exhaust gas. Catalyst 245 may be a membrane reactor or packed-bed reactor, as discussed below in connection with FIG. 5.

As stated above, various catalytic materials may be used to coat the catalyst surfaces or to coat the surface of the heat exchanger, for purposes of the steam reformation reaction. Specific examples of catalyst materials are nickel, platinum, palladium, and rhodium. Catalyst supports can include cerium oxide, aluminum oxide, and silicon oxide.

Many other configurations for circulating exhaust from the main exhaust line 119 within catalyzed heat exchanger 180 are possible. A multiplicity of straight tubes or U-shaped tubes or coils could be used. These various configurations could be used to heat the internal portions of reactor 245 and not simply its outer surface. As stated above, the catalyst material could be used as a coating on the heat exchanger tubing.

Referring again to FIG. 1, the exhaust from the main cylinders is treated with a three-way catalyst (TWC) or other exhaust aftertreatment device 120 installed on the main exhaust line 119. The placement of the catalyzed heat exchanger 180 may be downstream the TWC 120, such as when a significant exotherm is generated across the TWC. In other embodiments the catalyzed heat exchanger may be positioned upstream of the TWC, close to the engine so that the EGR gas temperature is maintained as high as possible. In other words, the placement of the catalyzed heat exchanger 180 may depend on the design of a particular engine.

Catalyzed Heat Exchanger with HPL EGR

FIG. 3 illustrates an engine 300 having a high pressure EGR loop, in which all cylinders 301 contribute to the EGR flow. In other words, engine 300 does not use dedicated EGR. The “take-off” point for the EGR loop 310 is from the exhaust manifold 303. In other respects, engine 300 is similar to engine 200.

Engine 300 has a catalyzed heat exchanger 180, which may be configured as shown in FIG. 2 or may have other of the above-described configurations. A fuel injector 185 provides hydrocarbon fuel for the steam reformation.

Catalyzed Heat Exchanger with LPL EGR

FIG. 4 illustrates an engine 400 having a low pressure EGR loop and non-dedicated EGR. In this EGR configuration, the “take-off” point for the EGR loop 410 is from the main exhaust line 419, downstream of turbine 404 b, but upstream aftertreatment device 420. In other respects, engine 400 is similar to engines 200 and 300.

Engine 400 has a catalyzed heat exchanger 180, which may be configured as shown in FIG. 2 or may have other of the above-described configurations. A fuel injector 185 provides hydrocarbon fuel for the steam reformation.

Catalyzed Heat Exchanger Performance

FIG. 5 illustrates conversion of methane (CH₄) as a function of temperature for both a membrane reactor (MR) and packed-bed reactor (PBR) as implementations of catalyst reactor 245. It is desired to maintain exhaust temperatures as high as possible for effective production of H₂ and CO during the steam reforming process. Because the steam reformation reaction is reversible in nature, continuous removal of H₂ from the gas feed (as is achieved with a membrane reactor) can result in increased yields of H₂ and CO. Because the steam reforming reaction is highly endothermic (206 kJ/mol HC), thermal energy is continuously removed from the gas stream as hydrocarbon fuel is converted to H₂ and CO. As described above, replacing thermal energy that is converted to chemical energy via transfer from the exhaust stream can increase the yield of H₂ and CO from the steam reforming reaction.

It is expected that for exhaust temperatures within the catalyst of the catalyzed heat exchanger 180 ranging between 773-923 degrees K, fractional conversion of methane can range from 0.4 to 0.9.

FIGS. 6 and 7 illustrate theoretical maximum H₂ and CO production, respectively, from catalyzed heat exchanger 180 as compared to other steam reforming configurations. The solid line represents H₂ and CO production with no heat exchanger, using only the thermal energy that is available in the EGR stream. The dashed line represents the use of an un-catalyzed heat exchanger and assumes doubling the thermal energy available in the EGR stream to drive the steam reforming reaction. The dotted line represents the case of catalyzed heat exchanger 180, where the catalyst surface temperature is maintained at the same value as the main exhaust stream gas temperature. This is the optimal case, and clearly results in the highest theoretical conversion of H₂ and CO. 

What is claimed is:
 1. A method of providing recirculated exhaust to an internal combustion engine having a number of cylinders, the cylinders receiving an intake charge via an intake line, with at least one cylinder being a dedicated EGR (exhaust gas recirculation) cylinder and the remaining cylinders being main cylinders whose exhaust is emitted via an exhaust manifold to a main exhaust line, comprising: providing an EGR loop to route recirculated exhaust from the dedicated EGR cylinder to a point on the intake line; installing a catalyzed heat exchanger on the EGR loop; wherein the catalyzed heat exchanger has a catalyst reactor and a heat exchanger; wherein the catalyst reactor is operable to perform a steam reformation process; installing a fuel injector on the EGR loop upstream of the catalyzed heat exchanger; receiving a hydrocarbon fuel via the fuel injector into the EGR loop upstream of the catalyzed heat exchanger; receiving exhaust from the main exhaust line into the heat exchanger and using the exhaust from the main exhaust line to heat the catalyst reactor; wherein the heat exchanger has at least one loop around the catalyst reactor, such a given portion of exhaust gas is heated simultaneously with the steam reformation process of that portion of exhaust, and the exhaust from the main exhaust line is used to keep the surface of the catalyst reactor at the same temperature as the main exhaust stream; and using the catalyzed reactor to increase the amount of hydrogen and carbon monoxide in the recirculated exhaust.
 2. The method of claim 1, wherein the hydrocarbon fuel is gasoline, diesel, methane, propane, or ethanol.
 3. The method of claim 1, wherein the hydrocarbon fuel is delivered from the same fuel reservoir as the used for the engine.
 4. The method of claim 1, wherein the catalyst reactor is a membrane or packed bed reactor.
 5. The method of claim 1, wherein the catalyst has catalyst material comprising nickel, platinum, palladium, or rhodium.
 6. The method of claim 1, wherein the engine has an exhaust aftertreatment device, and the catalyzed heat exchanger receives exhaust downstream of the aftertreatment device.
 7. The method of claim 1, wherein the engine has an exhaust aftertreatment device, and the catalyzed heat exchanger receives exhaust upstream of the aftertreatment device.
 8. A method of providing recirculated exhaust to an internal combustion engine having a number of cylinders, the cylinders receiving an intake charge via an intake line, and the cylinders emitting exhaust via a main exhaust line, comprising: providing an exhaust gas recirculation (EGR) loop to route recirculated exhaust from at least one cylinder to a point on the intake line; installing a catalyzed heat exchanger on the EGR loop upstream of the catalyzed heat exchanger; wherein the catalyzed heat exchanger has a catalyst reactor and a heat exchanger; wherein the catalyst reactor is operable to perform a steam reformation process; installing a fuel injector on the EGR loop upstream of the catalyzed heat exchanger; receiving a hydrocarbon fuel via the fuel injector into the EGR loop; receiving exhaust from the main exhaust line into the heat exchanger and using the exhaust from the main exhaust line to heat the catalyst reactor; wherein the heat exchanger has at least one loop around the catalyst reactor, such a given portion of exhaust gas is heated simultaneously with the steam reformation process of that portion of exhaust, and the exhaust from the main exhaust line is used to keep the surface of the catalyst reactor at the same temperature as the main exhaust stream; and using the catalyzed reactor to increase the amount of hydrogen and carbon monoxide in the recirculated exhaust.
 9. The method of claim 8, wherein the EGR loop is a low pressure EGR loop.
 10. The method of claim 8, wherein the EGR loop is a high pressure EGR loop.
 11. The method of claim 8, wherein the engine has at least one cylinder that is a dedicated EGR cylinder and wherein the EGR loop receives exhaust from the dedicated EGR cylinder.
 12. The method of claim 8, wherein the hydrocarbon fuel is gasoline, diesel, methane, propane, or ethanol.
 13. The method of claim 8, wherein the hydrocarbon fuel is delivered from the same fuel reservoir as the used for the engine.
 14. The method of claim 8, wherein the catalyst reactor is a membrane or packed bed reactor.
 15. The method of claim 8, wherein the catalyst has catalyst material comprising nickel, platinum, palladium, or rhodium.
 16. The method of claim 8, wherein the engine has an exhaust aftertreatment device, and the catalyzed heat exchanger receives exhaust downstream of the aftertreatment device.
 17. The method of claim 8, wherein the engine has an exhaust aftertreatment device, and the catalyzed heat exchanger receives exhaust upstream of the aftertreatment device. 